Thermal efficiency and weight are two of the primary concerns in the development of the Stirling engine. A Stirling engine is generally recognized as a hot gas engine of the type where pressurized gas is reciprocally displaced in a closed system between two spaces or chambers, one a hot chamber in which expansion may take place, the other a cold chamber in which compression may take place. Displacement of the closed gas (working gas) results in a temperature change generally at constant volume; expansion or compression takes place substantially at a uniform temperature. It is an engine which has two power strokes per piston and has an operation highly dependent upon the input of heat to the closed gas adjacent the high temperature chamber, typically on one side of a heat accumulator (regenerator) in the closed system.
Prior art constructions to date have utilized heater head assemblies whereby combusted gases or flue gases (typically in the range of 2,300.degree.-3,500.degree.F) are passed along a heat source system and about a heat transfer tube containing the closed gas and interconnecting the spaces. The mass flow of the heat source system varies considerably between idle and high speed operation of the engine. The amount of heat transferred to and through the walls of the tube and eventually to the closed gas (working gas) results in a reduction in the temperature of the combusted gases; the temperature typically is lowered to the range of 1,350.degree.-1,800.degree.F. Obviously a large amount of thermal energy remains within the heated medium (flue gas) after having passed about the heat transfer tube arrangement. To eliminate wasting the heat content, the prior art has turned to the use of a rotating regenerative wheel, usually of the ceramic type, which at one zone receives heat from the combustion gases and at another zone releases heat to the inducted air for preheating. The exhaust gases, after having passed through the wheel to give up considerable latent heat content, is usually in the temperature range of 650.degree.F. By the time the exhaust gases are finally released to atmosphere, they have assumed a temperature as low as 200.degree.-250.degree.F.
Unfortunately, the cost and weight added by the use of the regenerative wheel to utilize the latent heat of the spent gases is a problem. Many heater tube arrangements have been attempted by the prior art to overcome the basic problem.
One approach has been to increase the heat transfer capabilities by the use of finned tubing; smooth surfaces of the tubing is augmented by the use of flat fins which extend outwardly in a radiating direction of the center line of the tubing. Unfortunately, this presents a nesting problem for the tubing as well as a problem in the fabricating of the heater head assembly. One limitation on any solution will be location of the spaces. In a modern 4 piston Stirling engine the spaces are 90.degree. apart about the axis of the engine; the gases must be moved between these 2 spaces with efficiency and maximum heat exchange. To meet this limitation, a typical approach is to use a hair-pin bent tubing configuration between the high temperature space and the heat accumulator (regenerator); one leg of the configuration is shorter and generally parallel to the engine axis, the outer leg spirals about to meet the 90.degree. indexing and typically has auxiliary finned transfer surfaces.